Fiber reinforced composites have many applications in fields that require lightweight structure, fuel efficiency, and environmental durability in both the military and civilian sectors. In addition to the need for flame and moisture resistance, many military applications require that the materials have impact and ballistic resistant properties. Metals and ceramics are recognized to have good ballistic properties, but their densities are generally too high for applications that require mobility, ground, sea or air. Similarly, conventional polymeric foam cored sandwich structures are designed to enhance the bending stiffness of structures, but generally do not exhibit satisfactory ballistics resisance. Although such current foamed structures may have adequate impact properties for some applications, they do not have the required severe impact (ballistic) and flame resistant properties required for many military applications.
Current ballistic protective materials used for military vehicles and personnel shields include metals, ceramics, polymeric composites, and their combinations. Armor designs incorporating metals or ceramics with a backing material can successfully defeat armor piercing steel core projectiles, tungsten cores, and bomb fragments. However, the densities of these materials are normally too high for many applications including aircraft and portable protective shields. Conventional polymeric composites are significantly lighter in weight and generally offer good small arms protection. However, the demand for higher ballistic protection levels has increased due to the changes in the world scenario. New advanced materials and armor concepts must therefore be developed for military aircraft and personnel protection applications that offer higher protection levels, weight savings, and thus increased mobility and survivability of soldiers. Currently, for example, portable personnel protective shields used on the battlefield are made of metals. These structures are so heavy that they require several soldiers to move them. Anything this heavy cannot posses a mobility that is satisfactory for use in combat.
Many advanced materials have been developed to reduce the weight of combat vehicles and personal protective armor while maintaining ballistic protection limits. Current armor for combat vehicles includes a composite hull with add-on appliqué armor appropriate for the threat scenario. Armor designs containing ceramics are capable of meeting small arms ballistics requirements with substantial weight reduction from metallic structures. However, metallic and ceramic armor systems are both too heavy for personal protective shields that require mobility.
There are two major parameters that must be considered in designing armor, materials and structures. The first is the material system. The second is the architecture of the constituent materials. In the case of composites, they can be designed to resist, defeat, or deflect the ballistic projectile.
A wide variety of armor and armor systems have been designed and fabricated to in an attempt to meet the various land vehicle, personnel and aircraft armoring needs of the military. Some such examples are presented here. Metals, including titanium and aluminum alloys, have been shown to have higher ballistic resistance than boron/epoxy and graphite/epoxy composites, for example. However, even given the lightweight of the constituent materials they are generally very heavy and not suitable for extensive military aircraft applications. Ceramics are good for armor applications for ground vehicles. However, ceramics are very hard and brittle, which leads to fragmentation upon impact. Researchers have investigated an armor system comprised of a closed-cell aluminum foam layer between a ceramic tile strike-face layer and a composite backing plate. They noticed that the cells in the aluminum foam layers protected the armor by absorbing much of the impact energy and therefore delayed stress wave propagation through the armor. Researchers at Purdue University have constructed a multi-layered ceramic armor system that included a layer of either alumina or TiB2, a graphite plate, and a hard cover plate. This system demonstrated good ballistic performance, but was too heavy and brittle for aircraft structural applications.
Fiber reinforced polymer composites have demonstrated that they can replace metals, ceramics and other ballistic materials in many applications. The combination of some resins with lightweight ballistic fibers/fabrics has shown ballistic potential against, for example, fragmentation. Because of relatively simple fabrication procedures, complicated components can be fabricated using this class of materials. For example, conventional polymeric composites are good candidates for aircraft and small arms protection applications and have been investigated as such. Aramid/Phenolic/PVB is currently used for the fabrication of military soft armor like helmets. Other materials that have been evaluated for this purpose include Glass/Vinylester, Spectra®/Vinylester, Glass/Spectra®/Vinylester, Spectra®/Glass/Vinylester, etc. The results of testing of these materials indicate that the ballistic resistance of Glass/Spectra®/Vinylester is marginally higher than that of the Aramid/phenolic/PVB for soft armor applications. To our knowledge, Kevlar® and PBO fabrics are the state-of-the-art polymeric materials for hard armor applications. This material system has superior impact strength and a tight weaving style.
Monolithic ceramic targets routinely deliver better than twice the ballistic performance of their metallic counterparts for small arms projectiles traveling at muzzle velocities. Notwithstanding such robust performance, however; the same ceramics may be defeated by the same projectiles traveling at considerably lower velocities. This disturbing phenomenon is known as “shatter gap”. For example, aluminum oxide ceramic armor subjected to steel core .50 caliber M2 AP projectiles over a range of velocities exhibits “shatter gap” behavior.
In this type of ceramic armor system, at muzzle velocity, the defeat of the steel cored projectile is accomplished by blunting its rather pointed tip and shattering its steel core. Both of these core deformations are a result of the high surface hardness, modulus of elasticity and compressive strength of the ceramic. Rather than failing on impact, the ceramic maintains its surface to blunt the projectile's tip and reflects the high pressure shock wave from its rear surface back into the projectile. At the highest projectile velocities this shock pressure exceeds the ultimate strength of the core material and fractures the projectile into many pieces. Having distributed the forward momentum of the projectile among many smaller pieces, the defeat is easily accomplished by the armor back plate.
When the ceramic is impacted at considerably lower velocities, however, there may be insufficient pressure reflected back into the projectile to cause its shatter, and the projectile is left intact and nearly pristine. For a non-deformed core, defeat then depends on how much armor mass is there to stop it. An alumina ceramic impacted in this manner offers little resistance to penetration by a low velocity projectile, and the projectile easily parts the fractured pieces.
It is therefore apparent that there exists a continuing need for lightweight and improved armor systems for land vehicular, individual soldier and aircraft protection that provide the required ballistic resistance against a variety of threats all while being relatively light weight.